Apparatus and Method for Providing Amplifier Linearity Information

ABSTRACT

An apparatus for providing a linearity information associated with an amplifier includes an operating state determinator and an evaluator. The operating state determinator is configured to obtain information describing a gain of the amplifier for at least one bias condition of the amplifier. The evaluator is configured to obtain the linearity information based on both the information describing the gain of the amplifier and information about the at least one bias condition of the amplifier using a gain-bias characteristic of the amplifier. A bias circuit including the apparatus for providing the linearity information is also disclosed. A corresponding method for providing the linearity information includes: using the information describing the gain of the amplifier and the information about the at least one bias condition with the gain-bias characteristics to determine a relation of a current operating point described by the information with respect to the gain-bias characteristic; and deriving the linearity information from said relation.

FIELD

Some embodiments according to the invention are related to an apparatusfor providing linearity information associated with an amplifier. Someembodiments according to the invention are related to a bias circuit forproviding a bias condition to an amplifier on the basis of providedlinearity information. Some embodiments according to the invention arerelated to a method for providing linearity information associated withan amplifier.

BACKGROUND

In the following, some examples of possible applications for measuringor determining a degree of linearity of an amplifier will be described.

Amplifiers are widely used in nearly any kind of electronic device.Depending on the intended use of the amplifier, a suitable amplifierdesign can be selected from a wide variety of amplifier designsdeveloped and implemented to date. For example, an amplifier may be usedin an output stage in order to drive some kind of transducer such as aloudspeaker, an antenna, a mechanical actuator, or a light source. Anamplifier in an output stage is typically referred to as a poweramplifier (PA) and is classified as class A, B, AB, and C for analogdesigns, and class D and E for switching design. These classes differ,among others, with respect to a degree of linearity that can be achievedand also with respect to their efficiency. For example, class Aamplifiers are typically more linear than other types, but are veryinefficient. A class C amplifier on the other hand offers the advantageof high efficiency, but a disadvantage is high distortion, i.e. thedegree of linearity is relatively poor.

One field of technology in which PAs are widely used is the field ofmobile telecommunication to drive the antennas of base transceiverstations or mobile devices. In both cases, power efficiency is theconcern of the design process of the PA's. In the case of a PA used in abase transceiver station, the PAs make up a major part of the totalpower consumption, and thus often the operating costs of a basetransceiver station. In a mobile device, improving the efficiency of thePA is desirable in order to achieve a longer battery life and/or standbytime.

As mentioned above, improving the efficiency of a power amplifiertypically results in a degradation of the linearity. Moderntelecommunication standards, however, make relatively high demands onthe linearity of devices operating under these standards in order toreduce inter-modulation. An excessive inter-modulation typically resultsin the output signal of the amplifier having a spectrum that containsfrequencies which are not in the input signal, i.e. the spectrum isbroadened. Typically, this effect is undesired because it results in thegeneration of jamming signals for other telecommunication systems and awaste of bandwidth.

One way of saving current in a transmitter for mobile communication isto “reduce the bias” of the PA to the minimum allowing the system tooperate within the constraints dictated by the communication standard.All modulation types except GMSK (Gaussian Minimum Shift Keying) need acertain linearity to be guaranteed, therefore in an adaptive systemcapable of dynamically reducing the bias to control the consumption ofDC-power, it is desirable to provide an effective and easy way ofassessing the degree of linearity of the output signal.

Biasing in electronics is the method of establishing predeterminedvoltages and/or currents at various points of an electronic circuit toset an appropriate operating point. Linear circuits (or circuitsoperating in an approximately linear manner) involving transistorstypically require specific DC voltages and currents to operatecorrectly, which can be achieved using a bias circuit. The bias voltageapplied to a transistor in an electronic amplifier typically allows thetransistor to operate in a particular region of its trans-conductancecurve. Thus, the operating point and the corresponding bias voltageand/or current may preferably be selected so that the constraintssuggested or even dictated by the communication standard are met at arelatively low level of biasing. The operating point at a correspondingbias setting may be implemented in a fixed, non-variable manner. Thebehavior of the amplifier varies, however, as a function of differentvariables such as temperature, power amplifier mismatch, level of outputpower, bias, and power amplifier specimen. At present a preliminaryinvestigation of the signal-quality as a function of different variablesis performed at laboratory by precision instruments. A fixed table forthe best bias settings is then generated and used in the firmware of theproducts adopting that certain family of power-amplifiers.

SUMMARY

Some embodiments according to the invention provide an apparatus forproviding a linearity information associated with an amplifier.

An apparatus according to an embodiment of the teachings disclosed inthis document may comprise an operating state determinator configured toobtain information describing a gain of the amplifier for at least onebias condition of the amplifier. The apparatus may further comprise anevaluator configured to obtain the linearity information in dependenceon both the information describing the gain of the amplifier andinformation about the at least one bias condition of the amplifier usinga gain-bias characteristic of the amplifier.

In another embodiment according to the teachings disclosed herein, abias circuit for providing a bias condition to an amplifier comprises anapparatus for providing a linearity information and a bias conditioncontroller. The apparatus for providing a linearity information maycomprise an operating state determinator configured to obtaininformation describing a gain of the amplifier for at least one biascondition of the amplifier and an evaluator configured to obtain thelinearity information in dependence on both, the information describingthe gain of the amplifier and the information about the at least onebias condition of the amplifier using a gain-bias characteristic of theamplifier. The bias condition controller may adjust the bias conditionof the amplifier dependent on a value of the linearity informationdetermined by the apparatus for providing the linearity information.

An apparatus for providing a linearity information associated with anamplifier according to another embodiment of the teachings disclosedherein may comprise an operating state determinator and a function valuemapper. The operating state determinator may be configured to obtaininformation describing a gain of the amplifier for at least one biascondition of the amplifier. The function value mapper receives theinformation describing the gain of the amplifier and a correspondingbias condition. The function value mapper may be configured to map theinformation describing the gain and the corresponding bias condition toa corresponding value of the linearity information using a predeterminedgain-bias characteristic.

Another embodiment according to the teachings disclosed in this documentrelates to an apparatus for providing a linearity information associatedwith an amplifier, the apparatus comprising a means for obtaininginformation describing a gain of the amplifier for at least one biascondition of the amplifier and a means for obtaining the linearityinformation in dependence on both the information describing the gain ofthe amplifier and information about the at least one bias condition ofthe amplifier, using a gain-bias characteristic of the amplifier.

According to another embodiment, a method for providing a linearityinformation associated with an amplifier comprises: obtaininginformation describing a gain of the amplifier for at least one biascondition of the amplifier; obtaining information about the at least onebias condition; and obtaining the linearity information in dependence onboth the information describing the gain of the amplifier and theinformation about the at least one bias condition using a gain-biascharacteristic of the amplifier.

According to another embodiment of the teachings disclosed herein, amethod for providing a linearity information associated with anamplifier comprises: obtaining information describing a gain of theamplifier for at least one bias condition of the amplifier; obtaininginformation about the at least one bias condition; using the informationdescribing the gain of the amplifier and the information about the atleast one bias condition with a gain-bias characteristic to determine arelation of a current operating point described by the informationdescribing the gain and the information about the bias condition withrespect to the gain-bias characteristic; and deriving the linearityinformation from said relation.

An embodiment according to the teachings disclosed herein relates to acomputer readable digital storage medium having stored thereon acomputer program having a program code for performing, when running on acomputer, a method for providing a linearity information associated withan amplifier, the method comprising: obtaining information describing again of the amplifier for at least one bias condition of the amplifier;obtaining information about the at least one bias condition; andobtaining the linearity information in dependence on both theinformation describing the gain of the amplifier and the informationabout the at least one bias condition using a gain-bias characteristicof the amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments according to the teachings disclosed herein willsubsequently be described with reference to the enclosed figures, inwhich:

FIG. 1 shows a plot of the gain and the output power against availableinput power for different bias voltages;

FIG. 2 shows an enlarged region of the plot of FIG. 1 and some auxiliarymarkings;

FIG. 3 shows another enlarged region of the plot of FIG. 1 and differenttrajectories for constant output power and small signal gain;

FIG. 4 shows a plot of different gain-bias characteristics at differentoutput power and a corresponding threshold;

FIG. 5 shows a schematic block diagram of an architecture capable ofexploiting the teachings disclosed in this document;

FIG. 6 shows a schematic block diagram of an apparatus according to anembodiment of the teachings disclosed herein;

FIG. 7 shows a schematic block diagram of an apparatus according toanother embodiment according to the teachings disclosed herein;

FIG. 8 shows a schematic block diagram of an apparatus according to yetanother embodiment according to the teachings disclosed herein;

FIG. 9 shows a schematic block diagram of an apparatus according to afurther embodiment of the teachings disclosed herein;

FIG. 10 shows a schematic block diagram of an apparatus according toanother embodiment of the teachings disclosed herein;

FIG. 11 shows a schematic block diagram of an apparatus according to afurther embodiment of the teachings disclosed herein;

FIG. 12 shows a schematic block diagram of an apparatus according toanother embodiment of the teachings disclosed herein;

FIG. 13 shows a schematic plot of a gain-bias characteristic toillustrate a first option of using the gain-bias characteristic;

FIG. 14 shows a schematic plot of a gain-bias characteristic toillustrate a second option of using the gain-bias characteristic;

FIG. 15 shows a schematic plot of the gain-bias characteristic toillustrate a third option of evaluating the gain-bias characteristic;and

FIG. 16 shows a schematic flow diagram of a method for providing alinearity information according to the teachings disclosed herein.

DETAILED DESCRIPTION

In FIGS. 1 to 4 some relations between a gain, an input/output power,and a bias condition of a power amplifier will be illustrated andexplained. The findings and/or insights obtained by studying theserelations may be used in an apparatus for providing a linearityinformation according to the teachings disclosed herein.

FIG. 1 shows a combined plot of the gain G_(0P) and of the output powerP_(OUT) against available input power P_(AV) for different bias voltagesof a typical amplifier. The unit of the gain axis is dB. The units ofthe available input power axes and the output power axes are dBm. It canbe seen that up to an input power of approximately 0 dBm, the amplifierexhibits substantially linear behavior, since the gain is relativelyconstant at a value of approximately 15 dB and the output power P_(OUT)increases linearly with increasing input power P_(AV). In an input powerrange from 0 dBm and 10 dBm, the gain G_(0P) begins to decrease for someof the curves that correspond to relatively low bias levels. Forrelatively high bias levels on the other hand, the gain is substantiallyconstant in this region of the input power compared to input powerP_(AV) smaller than 0 dBm. For input powers P_(AV) greater thanapproximately 12 dBm, a significant reduction of the gain G_(0P) and acorresponding compression of the output power P_(OUT) can be observed,regardless of the bias level of the amplifier.

FIG. 1 thus shows the behavior of a real amplifier which is only linearwithin limits. Accordingly, the output power P_(OUT) increases with theinput power P_(AV) until a point is reached where some part of theamplifier becomes saturated and cannot produce anymore output; this iscalled clipping, and results in distortion. Before hard clipping occurs,a reduction in gain typically takes place in most amplifiers. Thereduction in the gain is called “compression”. In order to quantify thecompression effect, a 1 dB compression point may be determined which isdefined as the input power (or output power) where the gain is 1 dB lessthan the small signal gain.

Note that in FIG. 1, the curves corresponding to relatively high biaslevels are above the curves corresponding to lower bias levels. Notefurther that the gain of the amplifier is typically different fordifferent bias levels. Moreover, especially in the input power rangefrom 0 dBm to approximately 12 dBm, the rate of change of the gainG_(0P) over the available input power P_(AV) (i.e., the slope of thegain curve) is different for the different bias levels.

FIG. 2 shows an enlarged region of the plot shown in FIG. 1, inparticular the input power range between 0 dBm and 9 dBm in which therate of change of the gain G_(0P) over the input power P_(AV) exhibits astrong dependency on the bias level. The additional markings in FIG. 2illustrate what happens when the bias is reduced from a value VB1 to asecond value VB2, while the output power level is kept constant. It canbe seen in FIG. 2 that in order to maintain the output power constant(P1=P2), the available input power has to increase from P_(av1) toP_(av2), since the gain dropped from G1 to G2. It can also be seen inFIG. 2 that especially for low bias levels, the gain G_(0P) decreases asthe input power P_(AV) increases. The reason is that a relatively lowbias level provides little head room for the input signal so that theamplifier is pushed into compression earlier than for higher biaslevels.

In FIG. 3 which shows another enlarged region of FIG. 1, two differenttrajectories C′C′ and D-D′ are traced on the gain curves for twodifferent power output levels. For the trajectory C-C′ the poweramplifier remains clearly in linear operation. The gain curves arealmost horizontal and far from the gain compression points which liemore to the right where the gain curves drop down with a knee. Bylooking at the section D-D′ it is easy to notice that the first pointson the trajectory are in linearity and how the reduction of bias pushesthe PA more and more into compression as the operating point is movedtowards D′.

Let us also consider the small-signal gain and its variations with thebias conditions. This can be represented as a trajectory A-A′ in FIG. 3for a very low available input power P_(AV). By looking at the differenttrajectories and the different shapes of the gain curve for differentbias conditions, the following observations can be made: the variationof the gain with respect to a variation of the bias condition depends onthe available input power P_(AV) that is currently fed to the amplifier.Furthermore, as the available input power P_(AV) varies, the amount ofhow much the gain varies depends on the currently applicable biascondition. Note that the gain curves are all parallel where the PA isoperating in a substantially linear manner.

In FIG. 4, different gain trajectories are illustrated concurrently as afunction of the bias together with the small signal gain. It is possibleto see here that a threshold generated as an offset on the small signalgain can be set and used to assess the linearity degree of operation ofthe power amplifier. All the conditions above this threshold areaccepted while those points, where the compression level is too high,are under this threshold. The apparatus or the method for providing thelinearity information may thus determine whether the power amplifiercurrently operates in an acceptable operating condition. In case theapparatus or the method determines that the current operating conditionof the power amplifier falls in the region beneath the threshold, awarning may be issued or the bias condition for the power amplifier maybe adjusted. The uppermost curve (solid line with triangles andreferenced by “P_(out)=−10 dBm”) in FIG. 4 corresponds to the smallsignal case and corresponds to the trajectory A-A′ in FIG. 3. This caseis substantially equal to a case where the output power P_(OUT) is −10dBm. Fora relatively high bias condition of 8 V of e.g. a drain voltageor a drain-source voltage Vd of a MOSFET transistor used in a sourceconfiguration of a single-stage amplifier, a small signal gain of 15.1dB can be achieved, which corresponds to point A in FIG. 3. The point A′in FIG. 3 corresponds to a bias condition in which Vd=4 V. From FIGS. 3and 4, it can be seen that the small signal gain is 14.4 dB at point A′.

For the sake of comparison, let us evaluate a gain-over-bias curve for ahigh output power level of P_(OUT)=20 dBm. According to FIG. 4, the gainis also approximately 15.1 dB for the high bias condition (Vd=8 V). Withdecreasing bias conditions, the gain degrades rapidly down to 13 dB fora bias voltage of Vd=4 V.

In FIG. 4, the threshold is offset from the small signal case byapproximately 0.3 dB. Depending on the constraints imposed on thelinearity of the amplifier, a smaller or a larger offset may be used,e.g. 0.1 dB, 0.2 dB, 0.4 dB, or 0.5 dB. The threshold may be used by theapparatus or the method for providing the linearity information toevaluate whether the power amplifier is currently operating in anacceptable operating point in terms of linearity and/or bias condition.To this end, the information describing the gain of the amplifier andthe information about the bias condition may be evaluated against thethreshold by the apparatus and/or the method according to the teachingsdisclosed herein. Typically, the information describing the gain and theinformation about the bias condition may be regarded as forming a valuepair in a gain-bias plane. The threshold separates, in the gain-biasplane, a region of acceptable operating conditions from a region ofsub-optimal operating conditions. The evaluation of the value pairagainst the threshold may be done by a comparator or comparison. Thecomparator or comparison may either implement a two-dimensionalcomparison function (in the gain-bias plane), or one of the variables(gain or bias condition) may be used to adjust a threshold for the othervariable (i.e., bias condition or gain). When the apparatus or themethod according to the teachings disclosed herein uses a threshold, thelinearity information typically comprises a binary or discreteevaluation of the linearity of the power amplifier.

According to the teachings disclosed in this document, a way ofdetecting the quality of the signal at the output of a power amplifier(PA) by measuring the variation of the power gain of the power amplifierwhen the bias is changed will be presented in connection with thedescription relative to FIGS. 5 to 16. The proposed method is analternative to e.g. a method that is based on determining a peak-to-RMSratio.

The method described below and according to the teachings disclosedherein may be used, for example, in connection with an amplifier circuitin which the bias of the power amplifier can be dynamically varied, forexample, by means of a DC-DC converter and in which the output power canbe regulated by means of an open or a closed loop control or othersimilar control technique. Using an apparatus or a method as describedherein, it is possible to track the linearity of the power amplifierwhen comparing the actual gain with the small signal gain of the poweramplifier.

The determination of the linearity information according to theteachings disclosed herein in one embodiment is based on a measurementof the gain, therefore on measures of the average power of a signal,which means that there is no need to use a receiver in a feedback loopof the linearity measurement device. From feedback-design, it is knownthat it is typically easier to optimize a coupler for detection of theaverage power instead of for peak power. The coupler is typicallyarranged between an output of the amplifier and a load such as anantenna. Moreover, detection of the average power entails an integrationoperation which means that the signal-to-noise ratio will be higher ifcompared with measures on single points.

In a system where the output power level is constant, a variation of thegain corresponds to a variation on the input power level. This allowsthe tracking of the linearity just via measurement on the changes of thesetting of the input power to the PA with relatively low costs in termsof computational complexity and time and without comparing input andoutput signals.

To summarize the above, the linearity of a PA can be determined thanksto measures of the gain-changes or input-power changes to the PA, whichis exploited in the apparatus and the method for providing the linearityinformation. Suppose the PA is initially operated at a bias conditionwhich ensures linearity. This may be achieved by controlling a biascircuit of the power amplifier to operate according to this biascondition, e.g., a relatively high bias voltage or current. Thehypothesis of linearity is satisfied, as long as a preliminarycharacterization of the PA-family is run to fill in a table with “safe”bias values. The bias can subsequently be reduced in order to try andsave DC-power and the gain-variation is observed and compared with thatexpected from a small-signal. When it starts deviating by a certainthreshold, this means that the PA is being operated into compression.When a PA is being operated into compression, its gain is reduced forhigh input and/or output power, compared to the gain for smallerinput/output power. This dependency of the gain on the input/outputpower affects the linearity of the power amplifier. The allowed degreeof linearity of the output signal can be translated in an allowed degreeof compression of the PA which entails a safe threshold in thegain-variation curve. In order to obtain the linearity information, thecurrent operating condition of the power amplifier can be evaluatedagainst the threshold, i.e. whether the current operating condition isabove or beneath the threshold (or more generally: the relation of thecurrent operating condition with respect to the threshold). In theembodiments described below, this may be achieved by using a comparatoror a comparison action, which takes the operating condition and thethreshold as inputs and provides the discrete linearity information asan output. Note that the threshold may be a threshold function or athreshold table in case the operating condition is a multi variablequantity, e.g., the information describing the gain and the informationabout the bias condition.

In connection with transmitters for mobile-phones, one embodiment of themethod according to the teachings disclosed herein could be adoptedtogether with PAs which are sufficiently insensitive to load-variation,or where mismatches at the antenna reference plane are well isolatedfrom the PA-output reference plane. Another possibility would be to havea means of determining the mismatch, therefore, using this extra bit ofinformation to set an adaptive threshold (see FIG. 11 and thecorresponding descriptions for further details of such animplementation). If a trans-mission standard allows some empty slotsduring the communications, this time could be used to test thesmall-signal-gain and to adjust the threshold to changes of the matchingconditions (see FIG. 10 and the corresponding descriptions for furtherdetails regarding this aspect of the teachings disclosed herein).

FIG. 5 shows an example of realization of an architecture capable ofexploiting the teachings disclosed in this document. FIG. 5 shows anamplification chain as it could be used in a mobile phone, for example.A signal to be amplified is provided from the left to a transmitter Tr.The transmitter Tr processes the signal to be amplified which maycomprise frequency modulation and filtering, for example. The signalprocessed by the transmitter Tr is then output as an input signal for apower amplifier 110. The input signal has a certain signal power whichcorresponds to the input power for the power amplifier 110. The inputsignal power can be controlled by the transmitter Tr in dependence on acontrol signal or control variable k which the transmitter Tr receivesas a further input variable from a logic block L that will be explainedbelow. The power amplifier 110 amplifies the input signal according to acertain gain factor to produce an amplified signal. The amplified signalis supplied to an antenna. The power amplifier 110 is, in the caseillustrated in FIG. 5, isolated from the antenna by an isolator D, sothat the impedance seen by the PA 110 is constant for all operatingconditions, which entails that the small-signal-gain variation with biasis also fixed. The output path of the PA 110 is also coupled through afeedback B to a block A, whose functionality is that of generating asignal s proportional to the power of the coupled signal in order torepresent the output power of the PA 110. To the simplest extent theblock A could be a diode-detector. The block A provides the signal s tothe logic block L.

The logic block L will then set the power-level at the output of thetransmitter Tr (i.e. the power of the input signal for the poweramplifier 110) thanks to the control signal k provided to thetransmitter Tr so as to reach a targeted level of the signal s for acertain bias Vb. Accordingly, the output power of the power amplifier110 is, in this case, controlled by a closed loop control with thetargeted level of the power of the signal s as the set point and thecontrol signal k as the actuating variable of the control loop. Thelogic block L also provides the bias Vb or a bias control signal for thepower amplifier 110. Let us call ŝ, Vb1, kl the triplet of valuesreached. The algorithm described in this document can then beimplemented in the block L, as follows: For the target output powerlevel detected by ŝ, the bias Vb can be reduced by a desired step toreach the value Vb2. In order to maintain ŝ, k has to rise to the valuek2, i.e., the input power for the power amplifier 110 has to beincreased. Thus, a check on the variation of k, with respect to thevariation expected for small-signalconditions against a certainthreshold as in FIG. 4, leads to an estimate of the linearity of the PA110. This step of reducing the bias condition can be reiterated to reachthe desired threshold. In case of a further degradation of the linearityof the PA 110 being observed during the operation of the architecturedepicted in FIG. 5 by the logic block L, the logic block L may increasethe bias Vb again in a gradual manner or start over again at a safe biascondition.

By comparison to presently employed bias adjustment mechanisms apreliminary investigation of the signal-quality as a function ofdifferent variables is typically performed during the manufacturing ofan architecture as shown in FIG. 5. A fixed table for the best biassettings is then generated and used in the firmware of the productsadopting that certain family of power-amplifiers. It has been found thatthis table entails an overhead, namely that an extra-current-cost isgenerated by the inability of the system to adjust itself adaptively onthe actual signal-quality. At least some of the teachings disclosedherein enable a (further) reduction of the bias condition andconsequently of the power consumption by observing operating parametersof the power amplifier indicative of the amplifier's gain and its biascondition, and by setting the bias condition to a setting for which thepower amplifier still operates in a sufficiently linear way, yet thebias condition is less power-consuming than in the fixed table case. Inother words, by dynamically reacting to a current operating condition ofthe power amplifier, a safety margin for the bias condition of the poweramplifier can be reduced while still operating the power amplifier in asubstantially linear operating region.

FIG. 6 shows a schematic block diagram of a first embodiment of anapparatus for providing linearity information according to the teachingsdisclosed herein. The apparatus 120 may be used within the architectureas illustrated in FIG. 5, for example within the logic block L. The PA110 receives an input signal to be amplified at an input of the PA 110.The input signal has an input power P_(IN) which corresponds by andlarge to the available input power P_(AV) illustrated in FIGS. 1 to 3.The input power P_(IN) typically considers an AC portion of the inputsignal only while neglecting any quasi-DC components of the input signalcaused by a bias voltage or a bias current applied to the input signal.At an output of the PA 110, an amplified output signal is provided withan output power P_(OUT).

The apparatus 120 for providing the linearity information has two inputswhich receive information relative to the PA 110, such as theinformation describing the gain of the PA and the information about thebias condition. The box around the PA 110 drawn in dashed linerepresents circuitry and/or variables that are involved in controllingthe operation of the PA 110. The information received from theenvironment of the PA 110 are supplied internally to the apparatus 120to an operating state determinator 122 and a bias condition determinator124. The operating state determinator 122 is configured to obtaininformation describing a gain of the amplifier 110 for at least one biascondition of the amplifier. The bias condition determinator 124 isconfigured to obtain information describing a currently operating biascondition of the amplifier 110. For example, the bias conditiondeterminator 124 may comprise a lowpass filter. The operating statedeterminator 122 provides the information describing the gain of theamplifier 110 as a value which is a function of the gain f(g). The valuemay be either the gain g itself or another quantity that is related tothe gain of the amplifier 110, such as its input power P_(IN), itsoutput power P_(OUT), the control parameter k of FIG. 5, or anothersuitable quantity. The bias condition determinator 124 outputs aquantity h(bias) which is a function of the current bias condition, suchas a bias voltage or a bias current. Note that although the input of theapparatus 120 is shown as being directly connected to the input of thePA 110 in FIG. 6, this is not necessarily the case. Rather, a couplercould be provided. Note as well that in the embodiment shown in FIG. 6,the information f(g) describing the gain of the PA 110 is likely to beinput power P_(IN) for the PA 110. It is thus assumed that the outputpower P_(OUT) is otherwise known, for example, because it is regulatedby an open control loop or a closed control loop (not depicted).

The information f(g) describing the gain of the PA 110 and the quantityh(bias) are provided to an evaluator 126 which is configured to obtainthe linearity information in dependence on both inputs provided to theevaluator 126 from the operating state determinator 122 and the biasdeterminator 124. The evaluator 126 also evaluates a gain-biascharacteristic 128. The gain-bias characteristic 128 corresponds to anexpected gain-bias characteristic or a default gain-bias characteristicof the amplifier 110. By qualitatively or quantitatively comparing anoperating condition defined by the actual information describing thegain f(g) and the quantity describing the bias condition h(bias) with anoperating condition defined by the expected gain-bias characteristic128, the evaluator 126 may assess a currently valid degree of linearity.Thus, the evaluator 126 is configured to obtain the linearityinformation which may be used by other circuits or function blocks ofe.g. a transmitter of a mobile device in a telecommunication network.

FIG. 7 shows a schematic block diagram of a second embodiment of anapparatus 220 for providing a linearity information associated with theamplifier 110. By comparison to the first embodiment shown in FIG. 6,the operating state determinator 222 of the apparatus 220 according tothe second embodiment has two inputs, one for the input power P_(IN) andfor the output power P_(OUT). The operating state determinator's inputfor the input power P_(IN) is connected to a coupler arranged at aninputside of the power amplifier 110. The operating state determinator'sinput for the output power P_(OUT) is connected to a coupler arranged atan output side of the power amplifier 110. Other implementations forobtaining information about the input and output power are also possibleand are contemplated by the present invention. The information f(g)describing the gain of the amplifier 110 may be obtained by evaluating aratio of the output power P_(OUT) and the input power P_(IN). Thepurpose of the operating state determinator 122, 222 typically is toobtain a value for an average gain of the amplifier 110 over severalperiods or cycles of the input signal (or at least one period or cycleof the input signal). Therefore, the operating state determinator 122,222 (and other operating state determinators described below withrespect to subsequent figures), are typically not required to evaluatean instantaneous power of the input signal and/or the output signal ofthe amplifier 110. This fact typically simplifies an implementation ofthe operating state determinator.

The embodiment illustrated in FIG. 7 does not depend on the output powerP_(OUT) being regulated by means of a control loop. Rather, the inputpower P_(IN) and the gain of the amplifier 110 could vary in anarbitrary manner, resulting in a corresponding output power P_(OUT).

Generally, the information describing the gain of the amplifier maycomprise at least one of an information about a gain factor of theamplifier directly, an information about an input power P_(IN) of theamplifier, and an information about an output power P_(OUT) of theamplifier 110. Furthermore, the information describing the gain may be atemporal average over a selected time span. Typically, the selected timespan is longer than one period of the input signal, possibly by one orseveral orders of magnitude.

FIG. 8 shows a third embodiment of an apparatus 320 for providing thelinearity information which is capable of adjusting or calibrating thegain-bias characteristic 328 to match the specific power amplifier 110that is being used. The operating state determinator 122, the biascondition determinator 124, and the evaluator 126 are substantiallyidentical or similar to the corresponding components shown in FIG. 6 anddescribed in connection with FIG. 6. The apparatus 320 shown in FIG. 8further comprises a reference value determinator 334 and a biascondition comparator 332. The reference value determinator 334 isconfigured to determine a first value of the information describing thegain corresponding to an operating state of the amplifier 110 in whichthe amplifier operates in a substantially linear mode. The referencevalue determinator 334 is further configured to adjust the gain-biascharacteristic 328 so that the gain-bias characteristic substantiallycomprises the first operating point described by the first value and acorresponding bias condition, i.e., the gain-bias characteristic isshifted by the reference value determinator 334 so that the gain-biascharacteristic passes through the point in the gain-bias plane definedby the first value of the information describing the gain and thecorresponding information about the bias condition. To this end, thereference value determinator 334 comprises an input for the informationdescribing the gain of the amplifier as output by the operating statedeterminator 122. The reference value determinator 334 further comprisesan input for the information about the at least one bias condition asprovided by the bias condition determinator 124.

In order for the reference value determinator 334 to know whether theamplifier 110 currently operates in the substantially linear mode, alinearity criterion may be evaluated. To this end, the bias conditioncomparator 332 is configured to compare the information about the atleast one bias condition with a bias condition threshold that ensures apredetermined linearity. Recall from the discussion of FIG. 4 that arelatively high degree of linearity is obtained fora large range ofinput and output power values if the bias of the power amplifier 110 issufficiently high. A comparison result of the bias condition comparator332 indicates to the reference value determinator 334 whether theamplifier 110 operates in the substantially linear mode. When this isthe case, i.e. the bias condition comparator 332 outputs thecorresponding comparison result, a determination of the first value ofthe information describing the gain is triggered and the reference valuedeterminator 334 samples the currently valid information describing thegain of the amplifier and also the currently valid information about thebias condition as provided by the operating state determinator 122 andthe bias condition determinator 124, respectively.

The gain-bias characteristic 128, 328 is typically influenced byexternal and/or environmental parameters, such as temperature, supplyvoltage of the amplifier 110, amplifier mismatch, amplifier specimen,etc. The reference value determinator 334 and the bias conditioncomparator 332 allow to compensate for a systematic offset of thegain-bias characteristic 328 caused by external and/or environmentalinfluences as mentioned above. While it is likely that theenvironmental, external influences also modifies the shape of thegain-bias curve, it may typically be reasonable to assume that adeviation between the expected gain-bias characteristic and an actualgain-bias characteristic caused by a modification of the shape of thecurve is substantially negligible compared to a global offset of thegain-bias characteristic 328. Thus, the gain-bias characteristic may beadjusted to match a specific amplifier 110 by using a template gain-biascharacteristic and applying an offset to this template gain-biascharacteristic. This adjusting may be achieved by the reference valuedeterminator 334 in connection with the bias condition comparator 332 asillustrated and explained in the exemplary embodiment of FIG. 8.

Thus, in case the bias condition happens to be sufficiently high for theamplifier 110 to operate in the substantially linear mode regardless ofa level of the input power P_(IN) and the output power P_(OUT), thegain-bias characteristic 328 may be shifted up or down so that it passesthrough the currently valid operating point indicated by the referencevalue determinator 334, which in turn has received the currently validoperating point from the operating state determinator 122 and the biascondition determinator 124. As can be seen in FIG. 4, if the biasvoltage Vd is greater than 7 V, the gain-bias characteristics for alarge range of the output power level (from −26 dB to 20 dB) are allwithin a relatively narrow range for the gain of approximately 0.1 dB.The region where the gain-bias characteristics for various output powerlevels close in or are bundled in a relatively narrow gain range can beexploited by the reference value determinator 334. A template gain-biascharacteristic may be determined by e.g. the manufacturer of theamplifier 110 and stored in a memory that is provided within theapparatus 320 or accessible by the apparatus 320. Such a pre-storedgain-bias characteristic is believed to be sufficiently accurate atleast for amplifiers 110 belonging to the same family of amplifiers. Thetemplate gain-bias characteristic may then be shifted in dependence onthe currently valid information describing the gain of the amplifier andthe currently valid information about the bias condition so that theshifted template gain-bias characteristic passes through an operatingpoint described by these currently valid information. In this manner,the gain-bias characteristic may be adjusted to a particular amplifierat hand and/or to varying environmental conditions.

FIG. 9 shows a schematic block diagram of a fourth embodiment of anapparatus 420 for providing the linearity information associated withthe amplifier 110. In this embodiment, the bias condition of theamplifier 110 may be set by the apparatus 420 so that the apparatus 420may control the linearity of the amplifier 110. In FIG. 9, a biascircuit 412 is schematically illustrated as being connected to the inputof the amplifier 110. The following types of bias circuits may be usefulif the amplifier 110 comprises a bipolar transistor: fix bias (basebias), collector-to-base bias, fixed bias with emitter resistor, voltagedivider bias, and emitter bias. For a field effect transistor, a typicalbias circuit may, for example, set the drain voltage V_(d) to a specificvalue defining the operating point of the field effect transistor.Generally, biasing methods may modify the operating point of an inputsignal to the amplifier 110 by means of a DC offset and/or set aspecific value of a supply voltage of the amplifier 110.

The apparatus 420 comprises the elements of the apparatus 120 depictedin FIG. 6, that is, the operating state determinator 122, the evaluator126, and an entity for providing the gain-bias characteristic 128. Inaddition, the apparatus 420 comprises a bias condition controller 414which is configured to change the bias condition of the amplifier forcausing the amplifier to attain at least two different values of theinformation describing the gain of the amplifier. The bias conditioncontroller 414 is connected to the bias circuit 412 in order to set thebias circuit 412 to one of at least two different bias conditions. Tothis end, the bias circuit 412 may, for example, comprise one or morevariable components such as resistors, capacitors, or the like to set orregulate a bias voltage or a bias current, such as a base-emittervoltage V_(BE), a gate-source voltage V_(GS), a collector-emittervoltage V_(CE), a drain-source voltage V_(DS), a collector currentI_(C), and/or a drain current I_(D). It is also possible that the biascircuit 412 comprises one or more switches that are controlled by thebias condition controller 414. The switch(es) would then select amongtwo components having different nominal values to be activated forproducing a specific bias condition of the amplifier 110. Another optionwould be to selectively connect, disconnect, or short-circuit a certaincomponent such as a resistor or a capacitor in order to change theconfiguration of the bias circuit 412.

The bias condition controller 414 may periodically adjust the biascondition for the amplifier 110. The bias condition controller 414 maystart off at a relatively high bias condition which typically ensures asubstantially linear operation of the amplifier 110. Hence, the biascondition controller 414 may be configured to set the bias condition toa comparatively high bias value and (subsequently) to a lower biasvalue. The evaluator 126 may be further configured to determine adeviation between a) a value describing the gain of the amplifier at thelower bias value, and b) a previously determined value describing thegain of the amplifier at the lower bias value for a small-signal case(see FIG. 15 and corresponding description). Furthermore, the evaluator126 may also be configured to determine the linearity information as afunction of the determined deviation. The evaluator 126 may exploit theat least two different values of the information describing the gain ofthe amplifier 110 and the at least two corresponding bias conditions.

The bias condition controller 414 may be arranged for changing the biascondition in an iterative manner until at least one of the informationdescribing the gain and the bias condition reaches, or exceeds, athreshold defined by the gain-bias characteristic 128. Alternatively,the bias condition may be changed until an operating condition definedby an information describing the gain and an information about the biascondition reaches a predetermined region of the gain-biascharacteristic. To this end, the bias condition controller 414 mayexploit the linearity information, or a corresponding signal may be sentby the evaluator 126 to the bias condition controller 414.

FIG. 10 shows a schematic block diagram of a fifth embodiment of anapparatus 520 for providing the linearity information associated withthe amplifier 110. The apparatus 520 substantially fulfills thefunctionality of the apparatus 120 from FIG. 6, but is extended toproduce a small-signal gain-bias characteristic by itself. Thus, theapparatus 520 does not rely on a predetermined, stored small-signalgain-bias characteristic, but may determine the actual small-signalgain-bias characteristic of the amplifier 110 at hand. The small-signalgain-bias characteristic thus obtained may then be used within theentity providing the gain-bias characteristic 528 and, consequently,assist the evaluator 126 in providing the linearity information. Theapparatus 520 comprises a signal generator 516 which is configured togenerate a suitable small-signal input for the amplifier 410. Theselector 518 in the form of a two-way switch is connected to the inputof the amplifier 110. The selector 518 allows the input of the amplifier110 to be connected with a normal operating state input for a payloadsignal (designated by the reference sign P_(IN) for the power of theinput signal), or with an output of the signal generator 516. Theselector 518 is controlled by the evaluator 126 in the embodimentillustrated in FIG. 10. The evaluator 126 acts as a gain-biascharacteristic provider for providing a small-signal gain-biascharacteristic of the amplifier 110. By controlling the selector 518,the evaluator 126 knows when the suitable small-signal input is appliedto the amplifier 110 and can therefore derive the small-signal gain-biascharacteristic of the amplifier 110 by sampling value pairs of theinformation describing the gain of the amplifier and the informationabout the currently valid bias condition of the amplifier 110. Theinformation gathered in this manner, i.e. in the form of one or morevalue pairs, may then be used to adjust the gain-bias characteristic528. Some communication standards under which a mobile phone or a basetransceiver station operate, define time slots in order to organize thetransmission from different transmitters. Therefore, a transmittertypically has some idle time. This idle time may be used to set theselector 518 to the configuration in which the small-signal input isapplied to the amplifier 110. As it is a small signal, the output powerP_(OUT) of the amplifier 110 will not be very high and therefore willnot produce a strong output signal that could be radiated by an antennaconnected to the amplifier 10. Thus, probing the amplifier 110 with thesmall-signal input will practically not result in jamming the remotetransmitter(s) that are scheduled to transmit during a specific timeslot.

The embodiment shown in FIG. 10 could be combined with the fourthembodiment shown in FIG. 9, so that each time the evaluator 126 controlsthe selector 518 to connect the small-signal input with the amplifier110, a new bias condition is set by the bias condition controller 414and the bias circuit 412. In this manner, the small-signal gain-biascharacteristic (illustrated in FIG. 4) of the amplifier 110 may bedetermined for a plurality of bias conditions.

FIG. 11 shows a sixth embodiment of an apparatus 620 for providing thelinearity information associated with the amplifier 110. The sixthembodiment takes into account a degree of mismatch between the amplifier110 and a load connected to the output of the amplifier when determiningthe linearity information. A high degree of mismatch may result in adegradation of the linearity of the amplifier 110 so a knowledge aboutthe current degree of mismatch may improve the accuracy of the linearityinformation. The apparatus 620 comprises a mismatch detector 642 whichis connected between an output of the amplifier 110 and a load that isfed by an output signal of the amplifier 110. The evaluator 626comprises a threshold adjustor 644 and a comparator 646. An output ofthe mismatch detector 642 is connected to the threshold adjustor 644 forusing the determined amount of mismatch for adjusting a thresholdgain-bias characteristic. Another input to the threshold adjustor 644 isprovided by the entity providing the gain-bias characteristic 128. Thethreshold adjustor 644 modifies a threshold that is evaluated by thecomparator 646 in dependence on a result provided by the mismatchdetector 642. For example, the higher the mismatch between the amplifier110 and the load, the larger a safety margin for the linearityinformation could be chosen and the threshold would be shifted to highergain values, towards the small-signal gain-bias characteristic. Thecomparator 646 typically compares at least one of the informationdescribing the gain and the information about the bias condition withthe threshold provided by the threshold adjustor 644.

FIG. 12 shows a seventh embodiment of an apparatus 720 according to theteachings disclosed herein which comprises provisions for obtaining theinformation describing the gain by means of measuring the input andoutput power of the amplifier 110. The apparatus 720 comprises an inputpower detector 722 for detecting an input power of the amplifier 110 andan output power detector 723 for detecting an output power of theamplifier 110. The operating state determinator 122 is connected to theinput power detector 722 and the output power detector 723. Theoperating state determinator 122 is further configured to derive theinformation describing the gain of the amplifier from at least one of aninformation about the input power and an information about the outputpower provided by at least one of the input power detector 722 and theoutput power detector 723, respectively. The inputs of the powerdetectors 722, 723 may be connected to couplers at an input side and anoutput side of the amplifier 110, respectively.

FIG. 13 illustrates a schematic plot of an expected gain-biascharacteristic. Furthermore, a current operating point of the amplifier110, defined by the information describing the gain of the amplifier andthe information about the bias-condition of the amplifier, is alsodepicted in FIG. 13. A relation of the current operating point withrespect to the expected gain-bias characteristic can be seen in FIG. 13as the difference of the gain between the current operating point andthe expected gain-bias characteristic. As could be seen in the contextof FIG. 4, this difference can be used within the evaluator 126 as ameasure for the linearity of the operation of the amplifier 110. Theexpected gain-bias characteristic may be regarded as corresponding to asubstantially linear operation of the amplifier 110. The evaluator 126may thus be configured to determine said difference and to derive thelinearity information from the determined difference between theexpected gain-bias characteristic and the current operating point.

FIG. 14 shows another schematic plot of the gain-bias characteristic.The upper thick line in the diagram of FIG. 14 shows the small-signalgain-bias characteristic of the amplifier 110 which is either known fromlaboratory measurements and stored for being used by the evaluator 126,or dynamically determined as described in some of the previousembodiments. Beneath the small-signal gain-bias characteristic, threethreshold lines are depicted which have substantially the same shape asthe small-signal gain-bias characteristic, but are offset from thelatter by a certain amount. These threshold lines may be regarded as“iso-lines” for the linearity information according to some scalarmeasure of the linearity of the amplifier 110. A first line correspondsto 90% linearity, a second line corresponds to 80% linearity, and athird line corresponds to 70% linearity. The linearity information canthus be derived from these lines by comparing the current operatingpoint with the various threshold lines to determine whether an operatingpoint defined by the information describing the gain and the informationabout the bias condition is in a region above or below one of the lines,or in a region between two of the lines. The linearity information isprovided in dependence on the results of the comparison(s) between thecurrent operating point and the threshold line(s).

A function value mapper may be used to evaluate a current operatingpoint of the amplifier 110 with respect to the plot shown in FIG. 14.The function value mapper may be a part of the evaluator 126 andconfigured to receive the information describing the gain of theamplifier 110 and a corresponding bias condition. The function valuemapper may further be configured to map the information describing thegain and the corresponding bias condition to a corresponding value ofthe linearity information (or to an acceptable/unacceptable information)using a predetermined (or previously set) gain-bias characteristic asillustrated in FIG. 14. A possible implementation of the function valuemapper is that the function value mapper evaluates in which region ofthe gain-bias plane the current operating point is located. The functionvalue mapper could alternatively determine the linearity information inan analytical manner using an approximated function which maps thegain-bias plane to the linearity information. Evaluating the operatingpoint against one or several threshold lines as they are shown in FIG.14 corresponds to subdividing the gain-bias plane into a plurality ofsub-regions in which the linearity is acceptable or not acceptable.

FIG. 15 shows another schematic plot of two different gain-biascharacteristics of the amplifier 110 for different output powers. Thefirst gain-bias characteristic is drawn in a dashed line and correspondsto a small signal gain-bias characteristic. The second characteristic isdrawn in dashed-dotted line and corresponds to a gain-biascharacteristic for a certain output power P_(OUT) of the amplifier 110.Both characteristics are approximately equal at an operating point (VB1,G₁). As the bias voltage is reduced from VB1 to VB2, the small signalgain-bias characteristic shows that the gain is also reduced to a valueG_(REF). For the non-small-signal case (dashed-dotted line) the gain hasdropped farther to a value G₂. Since the small-signal case correspondsto a substantially linear mode of operation of the amplifier 110, thedeviation between G₂ and G_(REF) can be used within the evaluator 126 todetermine the linearity information. In particular, the evaluator 126may receive the small-signal gain-bias characteristic from the entitythat provides the gain-bias characteristic 128, 328, 528. Then the valueof the gain-bias characteristic 128, 328, 528 is evaluated for a valueVB2 of the bias condition indicated by the information about the biascondition as received from the bias condition determinator 124 or thebias condition controller 414. The gain value G_(REF) thus obtained forthe small-signal case is then compared to the actual gain value G₂ asindicated by the information describing the gain which is provided bythe operating state determinator 122. The evaluator 126 may comprise acomparator or a subtractor that implements this comparison. Thecomparison may comprise the determination of a difference G_(REF)−G₂.The value VB1 of the biasvoltage corresponds to a relatively high biasvalue and the value VB2 corresponds to a lower bias value. The gainvalue G₂ corresponds to a value describing a gain of the amplifier atthe lower bias value. The gain value G_(REF) corresponds to a previouslydetermined value describing the gain of the amplifier at the lower biasvalue VB2 for a small-signal case. Thus, it is possible to determine thelinearity information as a function of the determined deviationG_(REF)−G₂ i.e., on the result of the comparison or the subtractionperformed by the comparator or the subtractor within the evaluator 126.The linearity information may then be output by the evaluator 126 andthe apparatus 120, 320, 520.

FIG. 16 shows a schematic flow diagram for a method of providing alinearity information associated with an amplifier. At an action 802,information describing a gain of the amplifier for at least one biascondition is obtained. Information about the at least one bias conditionis obtained during an action 804. The order of the actions 802 and 804may be inversed, or the actions 802 and 804 may be performedconcurrently.

At an action 806, the information describing the gain of the amplifierand the information about the at least one bias condition are used inconnection with a gain-bias characteristic to determine a relation of acurrent operating point (current in the sense of “at this time”, not inthe sense of “electrical current”) with respect to the gain-biascharacteristic. The current operating point is defined by theinformation describing the gain and the information about the at leastone bias condition. The determination of the relation between thecurrent operating point and the gain-bias characteristic may comprise ananalysis of whether the current operating point is right on, above, orbeneath the gain-bias characteristic. As an alternative, a distancebetween the current operating point and the gain-bias characteristicaccording to a suitable scalar measure may be determined.

On the basis of the determined relation, the sought linearityinformation is derived from said relation at 808. To this end, therelation may be interpreted or mapped to a measure of linearity. Themeasure of linearity may be a continuous, scalar measure or it may bediscrete measure with values such as “good linearity”, “averagelinearity”, “low linearity”, and “non-linear”.

Although some aspects have been described in the context of anapparatus, it is clear that these aspects also represent a descriptionof the corresponding method, where a block or device corresponds to amethod step or a feature of a method step. Analogously, aspectsdescribed in the context of a method step also represent a descriptionof a corresponding block or item or feature of a correspondingapparatus. Some or all of the method steps may be executed by (or using)a hardware apparatus, like for example, a microprocessor, a programmablecomputer or an electronic circuit. In some embodiments, some one or moreof the most important method steps may be executed by such an apparatus.

Depending on certain implementation requirements, embodiments of theinvention can be implemented in hardware or in software. Theimplementation can be performed using a digital storage medium, forexample a floppy disk, a DVD, a Blue-Ray, a CD, a ROM, a PROM, an EPROM,an EEPROM or a FLASH memory, having electronically readable controlsignals stored thereon, which cooperate (or are capable of cooperating)with a programmable computer system such that the respective method isperformed. Therefore, the digital storage medium may be computerreadable.

Some embodiments according to the invention comprise a data carrierhaving electronically readable control signals, which are capable ofcooperating with a programmable computer system, such that one of themethods described herein is performed.

Generally, embodiments of the present invention can be implemented as acomputer program product with a program code, the program code beingoperative for performing one of the methods when the computer programproduct runs on a computer. The program code may for example be storedon a machine readable carrier.

Other embodiments comprise the computer program for performing one ofthe methods described herein, stored on a machine readable carrier.

In other words, an embodiment of the inventive method is, therefore, acomputer program having a program code for performing one of the methodsdescribed herein, when the computer program runs on a computer.

A further embodiment of the inventive methods is, therefore, a datacarrier (or a digital storage medium, or a computer-readable medium)comprising, recorded thereon, the computer program for performing one ofthe methods described herein. The data carrier, the digital storagemedium or the recorded medium are typically tangible and/ornon-transitory.

A further embodiment of the inventive method is, therefore, a datastream or a sequence of signals representing the computer program forperforming one of the methods described herein. The data stream or thesequence of signals may for example be configured to be transferred viaa data communication connection, for example via the Internet.

A further embodiment comprises a processing means, for example acomputer, or a programmable logic device, configured to or adapted toperform one of the methods described herein.

A further embodiment comprises a computer having installed thereon thecomputer program for performing one of the methods described herein.

A further embodiment according to the invention comprises an apparatusor a system configured to transfer (for example, electronically oroptically) a computer program for performing one of the methodsdescribed herein to a receiver. The receiver may, for example, be acomputer, a mobile device, a memory device or the like. The apparatus orsystem may, for example, comprise a file server for transferring thecomputer program to the receiver.

In some embodiments, a programmable logic device (for example a fieldprogrammable gate array) may be used to perform some or all of thefunctionalities of the methods described herein. In some embodiments, afield programmable gate array may cooperate with a microprocessor inorder to perform one of the methods described herein. Generally, themethods are preferably performed by any hardware apparatus.

The above described embodiments are merely illustrative for theprinciples of the present invention. It is understood that modificationsand variations of the arrangements and the details described herein willbe apparent to others skilled in the art. It is the intent, therefore,to be limited only by the scope of the impending patent claims and notby the specific details presented by way of description and explanationof the embodiments herein.

1. An apparatus for providing a linearity information associated with anamplifier, the apparatus comprising: an operating state determinatorconfigured to obtain information describing a gain of the amplifier fora bias condition of the amplifier; and an evaluator configured to obtainthe linearity information based on both the information describing thegain of the amplifier and information about the bias condition of theamplifier using a gain-bias characteristic of the amplifier.
 2. Theapparatus according to claim 1, wherein the evaluator is furtherconfigured to determine a relation of a current operating pointdescribed by the information describing the gain of the amplifier andthe information about the bias condition with respect to an expectedgain-bias characteristic, and derive the linearity information from thedetermined relation.
 3. The apparatus according to claim 1, wherein thegain-bias characteristic comprises a small-signal gain-biascharacteristic of the amplifier.
 4. The apparatus according to claim 1,wherein the information describing the gain of the amplifier comprisesat least one of an information about a gain factor of the amplifier, aninformation about an input power of the amplifier, and an informationabout an output power of the amplifier.
 5. The apparatus according toclaim 1, wherein the information describing the gain comprises atemporal average over a selected time span.
 6. The apparatus accordingto claim 1, wherein the evaluator is further configured to compare theinformation describing the gain of the amplifier with at least one gainthreshold in order to grade the linearity information according to atleast two categories.
 7. The apparatus according to claim 6, wherein theevaluator is further configured to generate the at least one gainthreshold as an offset on a small signal-gain, wherein a condition is inan accepted category of the at least two categories if the informationdescribing the gain is above the at least one gain threshold, andwherein a condition indicates that a compression level of the amplifieris too high if the information describing the gain is under the at leastone gain threshold.
 8. The apparatus according to claim 6, wherein theat least one gain threshold is determined based on a small-signalgain-bias characteristic of the amplifier and is dependent on theinformation about the bias condition.
 9. The apparatus according toclaim 8, wherein the information describing the gain comprises aninformation describing an input power to the amplifier and wherein thelinearity information corresponds to an estimate of the linearity of theamplifier.
 10. The apparatus according to claim 6, wherein the at leasttwo categories of the linearity information comprise a comparativelyhigh linearity category and a comparatively low linearity category. 11.The apparatus according to claim 1, wherein the evaluator is furtherconfigured to determine an actual variation of the informationdescribing the gain of the amplifier between a first bias condition anda second bias condition, compare the actual variation with a referencevariation of the information describing the gain of the amplifierbetween the first bias condition and the second bias condition, andobtain the linearity information as a function of a deviation of theactual variation from the reference variation, wherein the referencevariation describes an allowable variation of the information describingthe gain for which a corresponding linearity information is within apredetermined acceptable limit.
 12. The apparatus according to claim 1,further comprising a reference value determinator configured todetermine a first value of the information describing the gaincorresponding to an operating state of the amplifier in which theamplifier operates in a substantially linear mode, and furtherconfigured to adjust the gain-bias characteristic so that the gain-biascharacteristic substantially comprises a first operating point describedby the first value and a corresponding bias condition.
 13. The apparatusaccording to claim 12, further comprising a bias condition comparatorconfigured to compare the bias condition with a bias condition thresholdthat ensures a predetermined linearity, wherein a comparison result ofthe bias condition comparator indicates to the reference valuedeterminator whether the amplifier operates in the substantially linearmode for triggering the determination of the first value of theinformation describing the gain.
 14. The apparatus according to claim 1,further comprising a bias condition controller configured to change thebias condition of the amplifier to cause the amplifier to attain atleast two different values of the information describing the gain of theamplifier, wherein the evaluator is configured to exploit the at leasttwo different values and at least two corresponding bias conditions. 15.The apparatus according to claim 14, wherein the bias conditioncontroller is further configured to set the bias condition to acomparatively high bias value and to a lower bias value, wherein theevaluator is further configured to determine a deviation between a) avalue describing the gain of the amplifier at the lower bias value andb) a previously determined value describing the gain of the amplifier atthe lower bias value for a small-signal case, and also furtherconfigured to determine the linearity information as a function of thedetermined deviation.
 16. The apparatus according to claim 14, whereinthe bias condition controller is configured to change the bias conditionin an iterative manner until at least one of the information describingthe gain and the bias condition reaches or exceeds a threshold definedby the gain-bias characteristic.
 17. The apparatus according to claim 1,further comprising a gain-bias characteristic provider configured toprovide a small-signal gain-bias characteristic of the amplifier,wherein the gain-bias characteristic provider is configured to providethe amplifier with a suitable small-signal input and observe acorresponding output for at least one bias level.
 18. The apparatusaccording to claim 1, further comprising: a mismatch determinatorconfigured to determine an amount of mismatch between an output of theamplifier and a load that is fed by an output signal of the amplifier;wherein the evaluator comprises a threshold adjuster configured to usethe determined amount of mismatch for adjusting a threshold gain-biascharacteristic.
 19. The apparatus according to claim 1, furthercomprising at least one of an input power detector for detecting aninput power of the amplifier and an output power detector for detectingan output power of the amplifier, wherein the operating statedeterminator is further configured to derive the information describingthe gain of the amplifier from at least one of an information about theinput power and an information about the output power.
 20. A biascircuit for providing a bias condition to an amplifier, the bias circuitcomprising: an apparatus configured to provide a linearity informationcomprising an operating state determinator configured to obtaininformation describing a gain of the amplifier for a bias condition ofthe amplifier; an evaluator configured to obtain the linearityinformation based on both the information describing the gain of theamplifier and the information about the bias condition of the amplifierusing a gain-bias characteristic of the amplifier; and a bias conditioncontroller configured to adjust the bias condition of the amplifierbased on a value of the linearity information determined by theapparatus for providing the linearity information.
 21. An apparatus forproviding a linearity information associated with an amplifier, theapparatus comprising: an operating state determinator configured toobtain information describing a gain of the amplifier for a biascondition of the amplifier; a function value mapper configured toreceive the information describing the gain of the amplifier and acorresponding bias condition, the function value mapper being configuredto map the information describing the gain and the corresponding biascondition to a corresponding value of the linearity information using apredetermined gain-bias characteristic.
 22. An apparatus for providing alinearity information associated with an amplifier, the apparatuscomprising: a means for obtaining information describing a gain of theamplifier for a bias condition of the amplifier; a means for obtainingthe linearity information based on both the information describing thegain of the amplifier and information about the bias condition of theamplifier using a gain-bias characteristic of the amplifier.
 23. Amethod for providing a linearity information associated with anamplifier, the method comprising: obtaining information describing again of the amplifier for a bias condition of the amplifier; obtaininginformation about the bias condition; and obtaining the linearityinformation based on both the information describing the gain of theamplifier and the information about the bias condition using a gain-biascharacteristic of the amplifier.
 24. The method according to claim 23,wherein using the gain-bias characteristic comprises determining adistance, in a gain-bias plane, between the current operating points andat least one previously determined reference operating point, in orderto derive the linearity information based on the distance.
 25. Themethod according to claim 24, wherein the reference operating pointcorresponds to a small-signal mode of operation of the amplifier. 26.The method according to claim 25, further comprising: setting the biascondition to a setting in which the amplifier exhibits a substantiallylinear behavior; changing the bias condition in order to reduce a biaslevel of the amplifier; observing a variation of the informationdescribing the gain of the amplifier in response to the changed biascondition; comparing the variation with an expected variation of theinformation describing the gain; and deriving the linearity informationfrom a difference between the variation of the information describingthe gain and a variation of the extracted information describing thegain.
 27. A computer readable digital non-transitory storage mediumhaving stored thereon a computer program having program code forperforming, when running on a computer, a method for providing alinearity information associated with an amplifier, the methodcomprising: obtaining information describing a gain of the amplifier fora bias condition of the amplifier; obtaining information about the biascondition; and obtaining the linearity information based on both theinformation describing the gain of the amplifier and the informationabout the bias condition using a gain-bias characteristic of theamplifier.